Taking the High Road

It's amazing how many piston pilots shy away from using the high-altitude capabilities of their airplanes. That's a shame, says AVweb's Mike Busch, because whether you fly a pressurized twin or a humble straight-leg Skylane, up high is almost always the best way to fly eastbound...and often westbound, too! Using actual performance and winds aloft data, Busch makes a compelling case for high-altitude flight, and offers some handy rules-of-thumb for picking an optimum cruise altitude.

By Mike Busch
, This article originally appeared in CESSNA PILOTS ASSOCIATION magazine and is reprinted here by permission. | March 3, 1998

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About the Author ...

Mike Busch is editor-in-chief of
AVweb, a member of the technical staff at Cessna Pilots Association, and in a
prior lifetime was a contributing editor for The Aviation Consumer and IFR
Magazine. A 6,000-hour commercial pilot and CFI with airplane, instrument and
multiengine ratings, Mike has been flying for 36 years and an aircraft owner
for 33. For the past 14 of those years, he's owned and flown a Cessna T310R
turbocharged twin, which he maintains himself. In his never-ending quest to
become a true renaissance man of aviation, Mike's on the verge of earning his
A&P mechanic certificate. Mike and his wife Jan reside on the central
coast of California in a semi-rural area where he can't get DSL or cable
TV.

I recently returned from a marvelous trip
from California to the Cayman Islands and return in my Cessna T310R, some 5,000
nautical miles and 27 flight hours in all. The outbound half of the trip was
flown entirely at FL190, and the first third of the return trip was done at
FL180. Although it was summertime and the winds aloft were relatively mild, they
did provide 20 to 30 knots of "help" all the way from Santa Maria, Calif., to
Key West, Fla. On the return flight a week later, the winds aloft were
essentially calm between Key West and Shreveport, La., at FL180. Heading west
from Shreveport, the headwinds became rather stiff, so the last two legs were
flown down low (12,000').

My good friend
Chris served as my copilot for this trip, and we swapped seats on each
successive leg of the trip. Although Chris is an experienced instrument pilot
and aircraft owner based on the west coast, I was surprised to discover that
this was the first time he'd ever flown at the Flight Levels (other than as a
passenger), or even at oxygen altitudes. He found it to be quite a revelation,
and by the time we got to Grand Cayman, Chris was making discreet inquiries
about the possibility of turbocharging his Bonanza.

When I made the same trip to the Caymans and back last year, my copilot was
another friend, Joe, who is also an experienced instrument pilot and aircraft
owner. Turned out that Joe had never done any high-altitude flying before,
either.

From what I've been able to tell, the great majority of piston pilots shy
away from using the high-altitude capabilities of their airplanes. Most pilots
of normally aspirated airplanes seem to confine most of their flying to
altitudes of 10,000' and below, and most pilots of turbocharged but
unpressurized airplanes have never flown them at the Flight Levels. In fact, a
surprising number of pilots of pressurized birds seem averse to flying much
above the low teens.

I suppose it shouldn't be all that surprising. When we go through primary
flight training, our instructors tend to take us just high enough to make sure
we don't hit anything. Even our student cross-countries tend to be short enough
that a lengthy climb doesn't make much sense. The same holds true when we train
for our instrument rating.

After that, our instructors turn us loose to commit aviation on our own
recognizance. Few of us ever receive formal instruction on how best to fly an
800-mile trip (much less a 5,000-mile one). So we tend to stay in our comfort
zone, which for most of us means four-figure altitudes.

That's a shame, because it's at the high end of the altitude spectrum that
most of our airplanes achieve their best efficiency  and in many cases, their
best speed as well.

High isn't just for turbos...

I'm not just talking about turbocharged airplanes, either. Most
normally-aspirated birds are perfectly capable of cruise altitudes well into the
teens, and even a fixed-gear Skylane can claw its way to FL180 when appropriate
 for instance, to top the weather or catch a big tailwind.

In fact, let's take a look at a plain-vanilla, fixed-gear,
normally-aspirated Skylane. Here's the "Range Profile" page out of the 182Q
Pilots Operating Handbook.

At a low altitude like 4,000', maximum cruise speed is 139 KTAS at 75% power.
Continue climbing until the airplane "runs out of throttle" at 8,000' and max
cruise climbs to 144 KTAS. That extra 5 knots will save you 9 minutes on an 800
NM trip when you take the extra climb into account. (5:38 instead of 5:47, big
deal!).

Continue climbing to 12,000' and max cruise drops back to 139 KTAS (same as
at 4,000'), but at a much more fuel-efficient 64% power (which is all you can
get at that altitude with wide-open throttle). The same 800 NM trip will take 6
more minutes at 12,000' than at 4,000' (5:53 to be exact) because of the longer
climb, but you'll burn a whopping 12 gallons less fuel in the process, and
increase your IFR range by a full hour and 130 NM!

How far can we take this? Stick a cannula in your nose and climb all the way
up to 16,000'  high enough to fly right over the highest part of the Rocky
Mountains IFR  and max cruise drops to a still-respectable 130 KTAS at a
miserly 53% power. Because it takes a Skylane nearly 40 minutes to climb from
sea level to 16,000' at max gross, the 800 NM trip will take a half-hour longer
than at 12,000' (6:23), but you'll save 20 gallons and increase IFR range by a
full two hours compared to our 4,000' benchmark.

Unless you just happen to like low-and-slow, there's no logical reason to
cruise a Skylane lower than 8,000' because doing so makes all the numbers worse:
cruise speed, trip time, and range. On the other hand, climbing to 10,000' or
12,000' will cost you a negligible amount of time, and reward you with
substantially lower fuel burn and increased range.

These calculations are all based on zero-wind, but in real life the winds
aloft are often a decisive factor in determining the best altitude to choose. If
you're headed eastbound, odds are you'll have a tailwind  and the higher you
fly, the better it'll be.

In wintertime, climbing up high to catch favorable winds can pay off
spectacularly. In the low-to-mid teens, 50 knot tailwinds are commonplace and a
70 or 80 knot tailwind is a real possibility. Even in summer, when the winds
tend to be relatively light, going high can pay off. Here are some typical
summer winds I just pulled off the weather wire:

Even in these docile summertime conditions, we can expect 10 to 15 knots more
tailwind component at 16,000' than at 8,000', which almost exactly offsets the
TAS advantage of the lower altitude (144K vs. 130K). By climbing up high on an
eastbound trip, we'll go just as fast, burn considerably less fuel, and increase
our IFR range nearly 400 NM!

During the winter, when the winds tend to be stronger, going high on
eastbound trips tends to be an even better deal, saving both time and fuel.

...but for turbos, it's even better

If you've got a turbocharger, the argument for flying high becomes much more
compelling, because the higher you fly in a turbo, the higher your speed, range
and efficiency  at least up to the low Flight Levels in most turbocharged
airplanes. These birds really shine up in the high teens and low twenties, and
pilots who don't take advantage of this capability don't know what they're
missing.

For example, take a look at the "Range Profile" page for my T310R.

Starting with 180 KTAS at sea level  which is nothing to write home about
for a 570 hp airplane  max cruise speed at 73.6% power steadily increases with
altitude to a relatively blistering 221 KTAS at FL200. (Above that altitude,
available power starts dropping off fairly rapidly.)

At the same time, range with IFR reserves climbs from 820 NM to 970 NM.
Naturally, trip time and fuel burn for the proverbial 800 NM trip both drop
accordingly  from 4:14 and 143 gallons at 5,000 to 3:45 and 125 gallons at
FL200.

Any of you who've ever heard me talk about powerplant management know that I
don't believe in pushing my engines this hard, particularly in view of the fact
that they're not intercooled. I almost always throttle back to between 60% and
65% power and settle for around 205 KTAS at FL200 at a miserly fuel burn of 26
gallons/hour, giving me a range of well over 1,000 NM with IFR reserves (or
1,200 NM if I fill my 20-gallon wing locker tank).

Once again, these figures assume no-wind conditions. Add in the wind on an
eastbound trip and the results can get downright exciting. In the winter, I've
seen my groundspeed edge above 300 knots from time to time. That's fun! During
the summer, on the other hand, I'm happy with 230 or 240 on the GPS readout.

Needless to say, you pay the piper going westbound. But if the winds aren't
too strong, it may still pay to go high rather than low. In my airplane, I gain
22 knots of true airspeed by climbing from 10,000' to FL200. So if the headwind
at FL200 is only 10 or 15 knots stronger than at 10,000' (which is usually the
case in summertime), higher is still better.

In wintertime, of course, westbound aircraft are all in the same boat, turbo
or non-turbo. We bounce along at the MEA, try not to look at the groundspeed
readout, hope the fillings in our teeth don't fall out, and think about how much
fun the eastbound part of the trip was (or will be).

Picking your altitude

Choosing what altitude to file can be done with scientific precision or by
rule-of-thumb. If you do your flight planning by computer (as I generally do),
the precise method comes with the territory.

One of my favorite flight planning tools is the one on CompuServe operated by
EMI Aerocorp, Inc. This sophisticated online system can automatically choose
your route, prepare your weather briefing, and file your flight plan. It's not
free, but it costs only a few bucks per trip and I think it does a far better
job than DUATS.

One nice feature of the EMI system is its ability to help you choose an
optimum altitude. You can give it a bunch of different altitudes to try, and in
seconds it'll spit back a nice little matrix showing what your trip time and
fuel burn will be at each one. Here's what EMI just gave me for a theoretical
950 NM flight from Wichita, Kansas, to Frederick, Maryland:

ALT/FL

TTE

FUEL

5000

5:01

843 LB

9000

4:54

804 LB

13000

4:47

796 LB

FL190

4:32

763 LB

No surprises here. Going eastbound, higher is invariably better. The only
real question here is whether to go at 13,000' without oxygen (admittedly
stretching the regs a bit), or to stick a tube up my nose and go at FL190,
thereby saving 15 minutes and 5.5 gallons of gas. Which I choose probably
depends whether my wife is going along or not.

Okay, how about the return trip westbound from Frederick to Wichita? Let's
see what EMI says this time:

ALT/FL

TTE

FUEL

6000

5:46

941 LB

10000

5:32

906 LB

14000

5:28

906 LB

FL200

5:20

885 LB

Here's a perfect example of what I was talking about earlier. With mild
summertime winds aloft, higher is quicker even going westbound. By strapping on
oxygen and climbing from 12,000' to FL200, I gain 10 minutes and 3.5 gallons of
gas. Is it worth it? Tough call. I'd probably say yes if the forecast calls for
bumps at 12,000' (I hate bumps!), no if it's expected to be smooth.

In a pressurized aircraft, of course, this is a no-brainer. Higher is
quicker, cheaper, and smoother. No tubes up the nose required. End of
discussion.

Of course, if the trip is a short one, climbing way up to the Flight Levels
may not make sense because by the time you get there, it'll be time to start
down. Consider, for example, the flight I plan to make tomorrow: Santa Maria,
California, to Las Vegas, Nevada  a distance of only 280 NM. Let's see what
the EMI system says:

ALT/FL

TTE

FUEL

9000

1:41

299 LB

13000

1:41

304 LB

15000

1:41

309 LB

FL190

1:42

315 LB

Sure enough, the trip's so short that climbing doesn't help, either in time
or fuel burn. So unless it's bumpy, I'll go low.

No computer? Use your thumb!

If you'd rather do flight planning the old fashioned way, or if you're caught
on-the-road without your laptop, how can you tell whether it's worth going high
or not? Actually, with a few simple rules of thumb, you can usually get close
enough. The following hack works pretty well for me:

"It doesn't usually pay to climb formore
than one-third of the trip."

For instance, my POH says that it takes 30 minutes for my T310R to
cruise-climb from sea level to FL200. Using the "one-third rule," it wouldn't be
worth climbing that high unless the total trip time is at least an hour and a
half. (Given the extra hassle of using oxygen, I'd probably raise that minimum
to two hours.) On the other hand, it only takes me 15 minutes to cruise-climb to
12,000', so that's a reasonable cruise altitude for trips as short as 45
minutes.

Suppose we're talking about a long trip that easily meets the requirements of
the "one-third rule." If our aircraft is turbocharged, we know that it flies
faster and more efficiently the higher we climb. If tailwinds are forecast, then
we want to go up high for sure  if we're not pressurized, the only question is
whether we are willing to go on oxygen (and that's probably a function of
whether we're carrying passengers, and how many). On the other hand, if the
forecast calls for headwinds, then the question becomes:

"Will the higher airspeed help usmore than
the headwind hurts us?"

That's easy enough to answer. For example, my airplane gains 22 knots of
cruise speed climbing from 10,000' to FL200. If the headwinds at FL200 are less
than 22 knots stronger than the winds at 10,000', chances are good that the
higher altitude will be better. The longer the trip and the less the additional
headwind, the better it will be.

What about a normally-aspirated airplane? Up to the highest altitude where
you can achieve the desired cruise power  for example, 11,500' if we're flying
a 182Q and like to cruise at 65% power  the rules are exactly the same as for
a turbo  the higher you climb, the faster you'll go. Above that altitude,
however, the airplane slows down as we continue to climb. Continuing to climb
beyond this altitude seldom makes sense if we have a headwind which increases
with altitude. But if a tailwind is forecast, the question now becomes:

"Will the lower airspeed hurt usmore than
the tailwind helps us?"

For example, the Skylane loses about 5 knots of cruise speed climbing from
11,000' to 15,000'. If we gain more than 5 knots of tailwind by climbing to
15,000', then chances are good that the higher altitude will be better. Once
again, the longer the trip and the stronger the additional tailwind, the better
it will be.

Repeat after me: high is good!

If you're one of those pilots who comes from the "I won't climb higher than
I'm willing to fall" school, you've got nothing to be embarrassed about. Believe
me you've got plenty of company. But you're also missing something good.

I'm typing this on my notebook computer from a hotel room in the
Wichita Airport Hilton. I flew here yesterday from my home base in California 
a distance of 1,150 NM  in 5 hours flat at FL190, nonstop. I've flown this
route a lot, and I usually plan on 6.5 hours of flying at 13,000' plus an
hour-long fuel-and-burrito stop at Farmington, New Mexico. But this time, my
preflight briefing indicated that the tailwinds in the low Flight Levels would
be 40 to 50 knots all the way, making a nonstop trip feasible with comfortable
IFR reserves. I couldn't pass up a deal like that! The view of the Grand Canyon
from three miles up was simply spectacular, the Rockies were afire with yellow
and orange foliage as I looked down on them, and I touched down at ICT with an
hour and fifteen minutes of fuel in the tanks.

Do yourself a favor: give high a try. It's cooler and smoother up there. Your
airplane flies faster and more efficiently up high. ATC will usually give you
direct to just about anywhere. You're above terrain, obstructions, and often the
weather and the ice. The view is usually terrific. So are the tailwinds, if
you're lucky enough to be going in the right direction. Try it...you just might
like it!

NOTE: Other articles by the author related to
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